In the course of treating and preparing subterranean wells for production, a well packer and one or more screens along with a service tool are run into the well on a work string, with the packer being set against a casing bore. In a gravel pack service operation, a service seal unit is mounted on the work string and is reciprocated relative to certain flow ports and sealing points within the packer bore to route service fluid through various passages. The service seal unit carries vertical and lateral circulation passages which, when aligned with ports formed in the packer, permit service fluid such as acids, polymers, cements and gravel laden liquids to be injected into the formation through the bore of the work string and into the outer annulus between the sand screen and the perforated well casing.
At the well site, the casing is perforated across the production zone to allow production fluids to enter the well bore. In some wells, lower and upper sand screens are installed in the flow path opposite the perforations in the casing. Packers are set above and below the sand screens to seal off the annular region where production fluids are permitted to flow into the tubing. The annulus around the screens is packed with relatively coarse sand or gravel to reduce the amount of fine formation sand reaching the screens. The gravel is pumped down the work string in a slurry of water or gel and is spotted directly under the packer or above each sand screen. Typically, a lower telltale screen is joined to a primary sand screen by a nipple having a polished bore. The polished bore of the nipple is sealed against a wash pipe which is connected to the lower end of the service seal unit.
To assist in spotting the gravel around the lower telltale screen and around the upper primary screen, the service seal unit must be retracted from its run-in position to a first lower circulating position in which the tail pipe is sealed within the polished bore of the screen connection nipple. In this position, gravel is pumped through the work string and through the bore of the service seal unit into the annulus between the screen and the casing. The gravel pack is deposited as gravel accumulates in the annulus around the lower telltale screen, with the gel or water carrying liquid being circulated upwardly through the wash pipe and through the annulus between the packer and the service seal unit into the annulus between the well casing and the work string, where it is returned to the surface.
After the gravel pack has been deposited around the lower telltale screen, the service seal unit is retracted further within the polished bore of the packer to an upper circulating position in which the seal between the wash pipe and the polished bore of the screen connection nipple is opened. In this position, the slurry is pumped down the work string through the flow ports of the service seal unit into the annulus between the tail pipe and the well casing. The coarse gravel accumulates above the lower gravel pack in the annulus between the upper screen and the perforated well casing. The water or gel is then circulated to the surface through the wash pipe and through the annulus above the packer between the work string and the well casing.
The success of the foregoing gravel pack operation is dependent upon the ability to retract the service seal unit an appropriate distance to position its lateral flow passages properly, and also to insure that the seal between the wash pipe and the polished bore of the screen connection nipple is established for the lower circulating mode, and is open for the upper circulating mode.